NREL Solar Cell Sets World Efficiency Record
at 40.8 Percent

Aug 13, 2008 - National Renewable Energy Laboratory
(NREL)

Scientists at the U.S. Department of Energy's
National Renewable Energy Laboratory (NREL) have set a world
record in solar cell efficiency with a photovoltaic device
that converts 40.8 percent of the light that hits it into
electricity. This is the highest confirmed efficiency of
any photovoltaic device to date.

The inverted metamorphic triple-junction solar
cell was designed, fabricated and independently measured
at NREL. The 40.8 percent efficiency was measured under
concentrated light of 326 suns. One sun is about the amount
of light that typically hits Earth on a sunny day. The new
cell is a natural candidate for the space satellite market
and for terrestrial concentrated photovoltaic arrays, which
use lenses or mirrors to focus sunlight onto the solar cells.

The new solar cell differs significantly from
the previous record holder – also based on a NREL design.
Instead of using a germanium wafer as the bottom junction
of the device, the new design uses compositions of gallium
indium phosphide and gallium indium arsenide to split the
solar spectrum into three equal parts that are absorbed
by each of the cell's three junctions for higher potential
efficiencies. This is accomplished by growing the solar
cell on a gallium arsenide wafer, flipping it over, then
removing the wafer. The resulting device is extremely thin
and light and represents a new class of solar cells with
advantages in performance, design, operation and cost.

NREL's Mark Wanlass invented the original
inverted cell, which recently won a R&D 100 award. His design
was modified by a team led by John Geisz that further optimized
the junction energies by making the middle junction metamorphic
as well as the bottom junction. Metamorphic junctions are
lattice mismatched – their atoms don't line up. The material
properties of the mismatched semiconductors allows for greater
potential conversion of sunlight.

NREL is the U.S. Department of Energy's primary
national laboratory for renewable energy and energy efficiency
research and development. NREL is operated for DOE by Midwest
Research Institute and Battelle.